C Pointers

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A very basic lecture on C pointers

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C Pointers

  1. 1. Pointers in C Omar Mukhtar
  2. 2. Outline  Review of concepts in previous lectures  Introduction to pointers  Pointers as function arguments  Pointers and arrays  Pointer arithmetic  Pointer-to-pointer
  3. 3. Review and Background  Basic data types − Place-holders for numeric data  Integer numbers (int, short, long)  Real numbers (float, double)  Characters / symbols codes (char)  Arrays − A contiguous list of a particular data type  Functions − Give “name” to a particular piece of code − Modularization & reuse of code
  4. 4. Pointers  The most useful and tricky concept in C language − Other high level languages abstract-out this concept  The most powerful construct too − Makes C very fast − Direct interaction with hardware − Solves very complicated programming problems
  5. 5. What are pointers?  Just another kind of “placeholder” to hold “address” of memory location − Address is also a number − Itself resides on some memory location Memory Address Value 0x8004 ... 0x8008 variable A 129 0x800C ... 0x8010 0x8008 address of A 0x8014 ...
  6. 6. What are pointers?  Declare variables a, b − int a, b;  Declare a pointer − int* pa;  Set value of a − a = 10;  Point pa to address of a − pa = &a;  Set value of a using pa − *pa = 12; pa = &b;
  7. 7. Pointer Operators  “address-of” operator: & − Gets address of a variable  De-referencing operator: * − Accesses the memory location this pointer holds address of
  8. 8. Pointers and Functions  A function can be passed arguments using basic data types − int prod(int a, int b) { return a*b; }  How to return multiple values from function? − void prod_and_sum(int a, int b, int*p, int* s) { *p = a*b; *s = a+b; }
  9. 9. In & Out Arguments of Function  A function may like to pass values and get the result in the same variables. e.g. a Swap function.  void swap(int* a, int* b) { int c; c = *b; *b = *a; *a = c; }  int a = 5; b = 6;  swap(&a, &b);  // a hold 6 and b hold 5 now.
  10. 10. Pointers and Arrays  Since arrays are a contiguous set of variables in memory, we can access them with pointers  int arr[5];  int *p = &arr[0];  *(p+0) = 1; // arr[0]  *(p+1) = 2; // arr[1]  *(p+2) = 4; // arr[2]  *(p+3) = 8; // arr[3]  ...
  11. 11. Pointer Arithmetic  Arithmetic operators work as usual on ordinary data types. − int a = 1; a++; // a == 2  It gets a bit complicated when arithmetic operators are used on pointers  int* p = 0x8004; p++;  What does p hold now? 0x8005???  Compiler knows that p is a pointer to integer type data, so an increment to it should point to next integer in memory. Hence 0x8008.
  12. 12. Pointer Arithmetic  So an arithmetic operator increases or decreases its contents by the size of data type it points to  int* pi = 0x8004; double* pd = 0x9004; char* pc = 0xa004;  pi++; // pi == 0x8008  pd++; // pd == 0x900c  pc++; // pc == 0xa005  Only '+' and '-' operator are allowed. '*' and '/' are meaningless.
  13. 13. Pointer-to-Pointer  Pointer variable is just a place-holder of an address value, and itself is a variable. − Hence a pointer can hold address of other pointer variable. In that case it is called a “double pointer”.  int*p; int **pp; pp = &p;  e.g a function may like to return a pointer value  void pp_example(int** p) { *p = 0x8004; }  int *p; pp_example(&p);
  14. 14. Pointer Pitfalls  Since pointer holds address of memory location, it must never be used without proper initialization.  An uninitialized pointer may hold address of some memory location that is protected by Operating System. In such case, de- referencing a pointer may crash the program.  An initialized pointer does not know the memory location, it is pointing to is, holds a valid value or some garbage.  A pointer cannot track boundaries of an array.

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